Displays that incorporate mechanical light modulators can include hundreds, thousands, or in some cases, millions of moving elements. In some devices, every movement of an element provides an opportunity for static friction to disable one or more of the elements. This movement is facilitated by immersing all the parts in a working fluid (also referred to as fluid) and sealing the fluid (e.g., with an adhesive) within a fluid space or gap in a MEMS display cell. The fluid is usually one with a low coefficient of friction, low viscosity, and minimal degradation effects over the long term.
Because the fluid may possess a thermal expansion coefficient which is different from that of the substrates which contain the fluid, variations in operating temperature can lead to strong variations in the fluid pressure within the display. These internal pressure variations can lead to bulging or warping of the display surface and in some cases to the formation of vapor bubbles within the display. For example, the CTE of the glass that may be used for a MEMS substrate may be about 3.5 ppm/K and the volumetric CTE for a suitable working fluid may be about 1200 ppm/K. Additionally, the CTE of the adhesive used to seal the fluid in the display can govern the expansion of the cell gap. In some embodiments, the CTE of the adhesive may be about 80 ppm/K. Thus, the working fluid in the gap in the MEMs display expands and contracts roughly 400 times more than that of the glass and 15 times more than that of the adhesive. For an approximately 50 degree Celsius temperature difference, the volume difference between the substrate and that of the fluid is about 5.5%. Thus, if the display is sealed at about 20° C. and is later heated to about 80° C., the fluid will expand about 5.5% more than the glass MEMS substrate, which is in turn dominated by the adhesive swelling or expanding. The differences in expansion cause a force to be exerted on the glass, resulting in a swelling of a portion of the display. This swelling amount is difficult to estimate accurately around the edges of the MEMs display because the glass MEMS substrate is confined around the edges by the adhesive and is generally free to deform in the center. In some embodiments, this swelling may in the center of the gap in the MEMS display may be as large as about 1.5 microns. A similar effect may occur as the MEMS display is cooled. If the display is cooled about 50° C., then the same fluid volume difference of about 5.5% will result.